Aimed at the problem that Bias and cycle slips of Carrier Phase measurements will occur because of multipath error, satellite signals blockage, satellite failure and other factors in utilizing differential carrier phase positioning, this paper proposes an integrity monitoring necessarily. The carrier phase integrity monitoring algorithm is researched based on single differential carrier phase measurements. After carrier phase inter ambiguities are determined, test statistic is constructed by using the method of least squares and threshold set according to the false alarm probability and number of observed satellites to test carrier phase measurements. Relationship between vertical protection level, satellite geometry and miss detection probability is analyzed. The experiment results indicate that when the false alarm probability is 10－7, the algorithm in this paper can detect measure bias effectively, and is more effective when the bias is as small as 0.4 cycle. When the missing alarm rate is 10－6 and the vertical protection level is 1.1 m, the algorithm meets the requirements of availability, and the addition of 1 observation satellites can reduce the vertical protection level by about 2 cm.